In the field of high-speed optical communications, an optical modulator (LN optical modulator) is currently used that uses an electro-optical effect of dielectric material consisting primarily of lithium niobate (LiNbO3) and the like. In order to make an optical modulator still smaller, a semiconductor optical modulator is starting to become common that uses an electroabsorption effect of semiconductors. An example of such semiconductor optical modulator is described in Japanese Patent Application Laid-Open Publication No. 2013-167703.
The related semiconductor optical modulator described in Japanese Patent Application Laid-Open Publication No. 2013-167703 is a Mach-Zehnder type optical modulator in which electrical signals are converted into optical signals, and includes a main Mach-Zehnder interferometer, a measurement unit, and a control unit.
The main Mach-Zehnder interferometer includes a first arm and a second arm that branch off from a main input optical path at one end, conjoin at the other end, and are connected to a main output optical path. The phase of the light propagating through each of the first arm and the second arm is varied depending on a voltage applied to a corresponding electrode of a first electrode and a second electrode, and the power of the light is attenuated.
The control unit controls the voltage applied to one of the first electrode and the second electrode based on the power of the light measured by the measurement unit so that the power of the light propagating through the first arm may become equal to the power of the light propagating through the second arm. The control unit controls the voltage applied to the other one of the first electrode and the second electrode according to an electrical signal.
It is said that, according to the related semiconductor optical modulator, the above-described configuration makes it possible to improve an extinction ratio of the light signal propagating through the main output optical path because the power of the light propagating through the first arm becomes equal to the power of the light propagating through the second arm.